It could be that the Ansys installation that you have does not include a licence for Fluent. It is the only option I see, but I don't have much experience with the workbench environment. I always start Fluent directly, for example through the windows start menu, is that something you can do?

It is the problem associated with serial key.. Are you facing this problem from the very first use? If yes, then you have to re-install your serial key.. otherwise go to the ANSYS licence manager(Server ANSLIC_ADMIN Utility), open it as administrator , then first STOP the ANSYS licence manager(probably 6th option) and after that START it.. restart your system.. try to open workbench..Hope it will work